CN113412239A

CN113412239A - Positive electrode active material for lithium secondary battery and method for preparing same

Info

Publication number: CN113412239A
Application number: CN202080013693.1A
Authority: CN
Inventors: 申昊锡; 金元泰; 柳淙烈; 林英根; 申先植; 河昇哲
Original assignee: LG Chem Ltd
Current assignee: LG Chem Ltd
Priority date: 2019-10-02
Filing date: 2020-09-29
Publication date: 2021-09-17
Anticipated expiration: 2040-09-29
Also published as: KR102430258B1; JP2022519233A; US20220106199A1; JP7230220B2; WO2021066574A1; EP3909916A4; EP3909916A1; CN113412239B; KR20210039963A; EP3909916B1

Abstract

The present invention relates to a method for preparing a positive electrode active material, the method comprising: a first step of adding a reaction solution to a reactor to form seeds of precursor particles, the reaction solution comprising a transition metal-containing solution containing a transition metal, an ammonium ion-containing solution, and an aqueous alkaline solutionAt least one of nickel, cobalt and manganese; by growing on the precursor particles up to the average particle diameter (D) of the precursor particles₅₀) The average particle diameter (D) of the finally prepared precursor particles₅₀) A second step of adding a carbon source to the reactor at 30% of the size of (a) to prepare carbon-introduced precursor particles; and a third step of mixing the carbon-introduced precursor particles with a lithium raw material and sintering the mixture at a temperature of 750 to 950 ℃ to prepare positive electrode active material particles, wherein the carbon introduced into the precursor particles is volatilized by the sintering of the third step to form cavities in the positive electrode active material particles, and the cavity ratio of the positive electrode active material is 5 to 20%.

Description

Positive electrode active material for lithium secondary battery and method for preparing same

Technical Field

Cross Reference to Related Applications

The present application claims priority from korean patent application No. 10-2019-0122545, filed on 2/10/2019, the contents of which are incorporated herein by reference.

Technical Field

The present invention relates to a positive electrode active material for a lithium secondary battery, a method for preparing the positive electrode active material, a positive electrode for a lithium secondary battery comprising the positive electrode active material, and a lithium secondary battery.

Background

As the technical development and demand for mobile devices increase, the demand for secondary batteries as an energy source has significantly increased. Among these secondary batteries, lithium secondary batteries having high energy density, high voltage, long cycle life and low self-discharge rate have been commercialized and widely used.

Recently, research into the increase in capacity and the reduction in charge/discharge time of such lithium secondary batteries has been actively conducted.

Lithium transition metal composite oxides have been used as positive electrode active materials for typical lithium secondary batteries, and among them, such as LiCoO₂The lithium cobalt composite metal oxide of (2) has a high operating voltage, and since lithium ions can be efficiently deintercalated during high-rate charging, the lithium cobalt composite metal oxide can react even at a high current, thereby providing a cathode active material having excellent charging efficiency. However, because of LiCoO₂The use of large amounts of LiCoO is being used due to the poor thermal properties due to the unstable crystal structure caused by delithiation and especially the use of expensive cobalt₂There is a limitation as a power source for applications such as electric vehicles.

Recently, with the rapid spread of electric vehicles, the performance of secondary batteries that can be used as power sources for medium and large-sized devices is considered to be very important. In particular, the main limitation of the electric vehicle is a long charging time compared to a vehicle having a conventionally used engine.

Therefore, it is required to develop a positive electrode active material for a secondary battery that is capable of rapid charging without deteriorating the performance of the secondary battery during rapid charging.

Documents of the prior art

(patent document 1) Korean patent No. 1395846

Disclosure of Invention

Technical problem

An aspect of the present invention provides a method of preparing a positive electrode active material capable of suppressing performance degradation of a secondary battery during rapid charging and improving output characteristics.

Another aspect of the present invention provides a positive electrode active material.

Another aspect of the present invention provides a positive electrode for a lithium secondary battery comprising the positive electrode active material.

Another aspect of the present invention provides a lithium secondary battery comprising the cathode.

Technical scheme

According to an aspect of the present invention, there is provided a method of preparing a positive electrode active material, the method including: a first step of adding a reaction solution to a reactor to form seeds of precursor particles, the reaction solution including a transition metal-containing solution containing at least one of nickel, cobalt, and manganese, an ammonium ion-containing solution, and an aqueous alkaline solution; by growing on the precursor particles up to the average particle diameter (D) of the precursor particles₅₀) The average particle diameter (D) of the finally prepared precursor particles₅₀) A second step of adding a carbon source to the reactor at 30% of the size of (a) to prepare carbon-introduced precursor particles; and a third step of mixing the carbon-introduced precursor particles with a lithium raw material and sintering the mixture at a temperature of 750 to 950 ℃ to prepare positive electrode active material particles, wherein the carbon introduced into the precursor particles is volatilized by the sintering of the third step to form cavities in the positive electrode active material particles, and the cavity rate of the positive electrode active material is 5 to 20%.

According to another aspect of the present invention, there is provided a positive electrode active material including a lithium transition metal oxide, and including a cavity in a region where a distance from a center of a particle of the positive electrode active material to a surface of the particle is 0.3R or more, when the distance from the center of the particle is R, and having a cavity rate of 5% to 20%.

According to another aspect of the present invention, there is provided a positive electrode for a lithium secondary battery, comprising the above positive electrode active material.

According to another aspect of the present invention, there is provided a lithium secondary battery including the positive electrode for a secondary battery.

Advantageous effects

According to the present invention, the surface area of the positive electrode active material particles is increased to maximize the contact area between the positive electrode active material and the electrolyte, so that the performance degradation of the lifetime and resistance characteristics can be minimized during rapid charging, and the output characteristics can be improved.

Drawings

Fig. 1 is a cross-sectional Scanning Electron Microscope (SEM) photograph of the positive active material precursor particles prepared in example 1;

fig. 2 is a cross-sectional SEM photograph of the positive electrode active material in example 1;

fig. 3 is a cross-sectional SEM photograph of the positive electrode active material precursor particles prepared in comparative example 1; and is

Fig. 4 is a cross-sectional SEM photograph of the positive electrode active material particles prepared in comparative example 1.

Detailed Description

Hereinafter, the present invention will be described in more detail.

It will be understood that the words or terms used in the specification and claims should not be construed as meanings defined in commonly used dictionaries, and will further be understood as having meanings that are consistent with their meanings in the context of the relevant art and technical idea of the present invention on the basis of the principle that the inventor can appropriately define the meanings of the words or terms to best explain the present invention.

The term "cavity" refers herein to an empty space formed when carbon introduced into the precursor particle volatilizes, which refers to a large pore having a diameter of about 10nm to about 1 μm, preferably about 20nm to about 500 nm.

The term "cavity ratio" herein refers to an area ratio occupied by a cavity in a cross section of the positive electrode active material particle, and the cavity ratio can be measured as follows: the positive electrode active material particles were cut by using a focused ion beam, and then cross-sectional photographs were taken using a scanning electron microscope, and then the ratio of the total area of the cavities to the cross-sectional area of the positive electrode active material particles was calculated using the cross-sectional photographs.

The expression "average particle diameter (D) herein₅₀) "may be defined as the particle size at 50% cumulative volume in the particle size distribution curve. For example, the average particle diameter (D) can be measured by using a laser diffraction method₅₀). Laser diffraction methods can typically measure particle sizes ranging from submicron to several millimeters, and can obtain highly repeatable and high resolution results.

Throughout this specification, TAP density refers to the apparent density of a powder obtained by vibrating a container under specific conditions when filled with the powder, and can be calculated by using a typical TAP density tester, and in particular can be calculated according to ASTM B527-06 by using TAP-2S (Logan Instruments Corp.).

Method for preparing positive electrode active material

The inventors have found that when a cathode active material precursor is prepared, a carbon source is added to a reactor at a specific time point to prepare carbon-introduced precursor particles, and the carbon introduced into the precursor particles is volatilized during sintering to form cavities in the cathode active material particles, so that output characteristics during rapid charging can be improved by maximizing the reaction between the cathode active material and an electrolyte, thereby completing the present invention.

Specifically, the method for preparing a positive electrode active material according to the present invention includes: a first step of adding a reaction solution to a reactor to form seeds of precursor particles, the reaction solution including a transition metal-containing solution containing at least one of nickel, cobalt, and manganese, an ammonium ion-containing solution, and an aqueous alkaline solution; by growing on the precursor particles up to the average particle diameter (D) of the precursor particles₅₀) The average particle diameter (D) of the finally prepared precursor particles₅₀) A second step of adding a carbon source to the reactor at 30% of the size of (a) to prepare carbon-introduced precursor particles; and a third step of mixing the carbon-introduced precursor particles with a lithium raw material and sintering the mixture at a temperature of 750 to 950 ℃ to prepare positive electrode active material particles. In this case, the sintering in the third step leadsThe carbon incorporated into the precursor particles volatilizes to form cavities in the positive electrode active material particles.

The positive electrode active material according to the present invention has a cavity ratio of 5% to 20%.

Hereinafter, a method of preparing a cathode active material according to the present invention will be described in more detail.

First, a reaction solution is added to a reactor to form seeds of precursor particles (first step), the reaction solution comprising: a transition metal-containing solution containing at least one of nickel, cobalt, and manganese; a solution containing ammonium ions; and an aqueous alkaline solution.

The transition metal-containing solution may contain a cation of at least one transition metal selected from the group consisting of: nickel, manganese and cobalt.

For example, the transition metal-containing solution may contain 40 to 90 mol% of nickel, 5 to 30 mol% of cobalt, and 5 to 30 mol% of manganese, and preferably contains 60 to 90 mol% of nickel, 5 to 20 mol% of cobalt, and 5 to 20 mol% of manganese.

The transition metal-containing solution may contain an acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, or oxyhydroxide of the above transition metal, and these materials are not particularly limited as long as they can be dissolved in water.

For example, it may be substituted with Ni (OH)₂、NiO、NiOOH、NiCO₃·2Ni(OH)₂·4H₂O、NiC₂O₂·2H₂O、Ni(NO₃)₂·6H₂O、NiSO₄、NiSO₄·6H₂Forms of O, fatty acid nickel salts, nickel halides, and the like contain nickel (Ni) in the transition metal-containing solution, and at least one of them may be used.

In addition, Co (OH) may be used₂、CoOOH、Co(OCOCH₃)₂·4H₂O、Co(NO₃)₂·6H₂O、Co(SO₄)₂·7H₂O or the like, cobalt (Co) is contained in the transition metal-containing solution, and of them, cobalt (Co) can be usedOne of them is less.

Further, manganese (Mn) may be contained in the transition metal-containing solution in the following form, and at least one of them may be used: oxides of manganese such as Mn₂O₃、MnO₂And Mn₃O₄(ii) a Manganese salts such as MnCO₃、Mn(NO₃)₂、MnSO₄Manganese acetate, manganese dicarboxylates, manganese citrates and manganese salts of fatty acids; oxyhydroxides, manganese chloride, and the like.

The basic aqueous solution may comprise at least one selected from the group consisting of: NaOH, KOH and Ca (OH)₂And water or a mixture of water and an organic solvent (specifically, alcohol, etc.) which can be uniformly mixed with water can be used as the solvent. In this case, the concentration of the aqueous alkaline solution may be 2M to 8M, preferably 3M to 5.5M. In the case where the concentration of the alkaline aqueous solution is 2M to 8M, precursor particles of a uniform size may be formed, and the formation time of the precursor particles may be fast and the yield may also be excellent.

The ammonium ion-containing solution may include at least one selected from the group consisting of: NH (NH)₄OH、(NH₄)₂SO₄、NH₄NO₃、NH₄Cl、CH₃COONH₄And (NH)₄)₂CO₃. In this case, water or a mixture of water and an organic solvent (specifically, alcohol or the like) which can be uniformly mixed with water may be used as the solvent.

Specifically, an alkaline aqueous solution and an ammonium ion-containing solution are first added to a reactor to adjust the pH to a range of 11 to 14, preferably 11.5 to 13, and then, a particle seed may be formed while the transition metal-containing solution is added to the reactor. In this case, since the pH value in the first reactor varies as the particle seed is formed by adding the transition metal-containing solution, the pH value can be controlled to be maintained at 11 to 14 by continuously adding the ammonium ion-containing solution and the alkaline aqueous solution and the transition metal-containing solution.

Then, the precursor particles are grown up to the average particle diameter (D) of the precursor particles₅₀) For the final preparationAverage particle diameter (D) of the precursor particles₅₀) Is 30%, a carbon source is added to the reactor to prepare precursor particles into which carbon is introduced (second step).

When the seed of the precursor particle is formed through the first step, the pH of the reaction solution is changed to grow the precursor particle.

In this case, the pH control in the reactor can be achieved by adjusting the injection amounts of the alkaline aqueous solution and the ammonium ion-containing solution. For example, the pH in the reactor may be adjusted to a range of 10 to 12.5, preferably 10.5 to 12, to grow the precursor particles.

Then, the average particle diameter (D) of the precursor particles₅₀) To the average particle diameter (D) of the finally prepared precursor particles₅₀) Is 30%, a carbon source is added to the reactor to prepare carbon-incorporated precursor particles.

For example, if the average particle diameter (D) of the finally prepared precursor particles₅₀) 2 μm to 20 μm, and preferably 5 μm to 15 μm, the average particle diameter (D) of the precursor particles₅₀) To 1 μm to 7 μm and preferably 2 μm to 5 μm, a carbon source is added to the reactor.

If the precursor particles are not grown in the first step, which is a step of forming precursor seeds to prepare precursor particles, and the carbon source is simultaneously added, the growth of the particles may be inhibited because the carbon source prevents aggregation of the precursor. In addition, if a carbon source is added at the start of the reaction, the cavity-forming region widens, thereby adversely affecting characteristics such as energy density.

According to the present invention, the carbon source may comprise at least one carbon-based material selected from the group consisting of: carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber and carbon nanotube, and an average particle diameter (D) of 10nm to 1 μm, preferably 20nm to 500nm can be used₅₀) The carbon-based material of (1).

If the average particle diameter of the carbon-based material contained in the carbon source satisfies the above range, the carbon source may be aggregated with the initial precursor particles and the precursor particles may grow to a desired size when synthesizing the precursor, and the carbon source may be present inside the finally prepared precursor particles of the positive electrode active material. If the average particle diameter of the carbon-based material is outside the above range, primary precursor particles smaller than the carbon-based material are attached around the carbon source, and thus, after sintering, no cavity is formed inside the positive electrode active material.

The carbon source may be added in such a manner that the carbon-based material is 5 to 20 vol%, preferably 5 to 15 vol%, with respect to 100 vol% of the entire precursor particles. If the content of the carbon source satisfies the above range, it is possible to contain a cavity having an appropriate volume inside the positive electrode active material after sintering while minimizing the loss of the tap density of the precursor particles. If the content of the carbon source is outside the above range and exceeds 20 vol%, cavities may be excessively formed, resulting in an excessive decrease in tap densities of the positive electrode active material precursor particles and the positive electrode active material particles. In addition, if the content of the carbon source is less than 5 vol%, the formation of the cavity is insufficient, and thus the effect of improving the lifetime and the resistance characteristics is insignificant during the rapid charging.

Finally, the precursor particles into which carbon is introduced and the lithium raw material are mixed and sintered at a temperature of 750 to 950 ℃ to prepare positive active material particles (third step).

For example, lithium carbonate (Li)₂CO₃) Or lithium hydroxide (LiOH) is used as the lithium-containing raw material, and the positive electrode active material precursor and the lithium-containing raw material may be mixed so that the molar ratio of the transition metal to Li becomes 1:1.0 to 1: 1.1. In the case where the lithium-containing raw material is mixed at a ratio less than the above range, the capacity of the prepared cathode active material may be reduced, and in the case where the lithium-containing raw material is mixed at a ratio greater than the above range, the preparation of the cathode active material may become difficult because the particles are sintered during the sintering process, the capacity may be reduced, and the separation of the cathode active material particles may occur after the sintering (causing the cathode active material aggregation phenomenon).

In addition, when the precursor particles into which carbon is introduced are mixed with the lithium raw material, a raw material containing a doping element (M) may be further included as necessary. In this case, M may comprise at least one selected from the group consisting of: al, Zr and W.

For example, at least one containing a metal element (M) selected from the following may be used as the raw material containing the metal element (M): acetate, nitrate, sulfate, halide, sulfide, hydroxide, oxide, or oxyhydroxide.

If the doping element (M) is further added, the effect of improving the lifetime and the output characteristics can be obtained.

According to the present invention, the precursor particles into which carbon is introduced, the lithium raw material, and the raw material containing the doping element (M) added as needed may be mixed, and the sintering process may be performed at a temperature of 750 to 950 ℃, preferably 800 to 900 ℃. In the case where the sintering temperature is less than 750 ℃, since raw materials may remain in the particles due to insufficient reaction, the high-temperature stability of the battery may be reduced, and the structural stability may be reduced due to the reduction in the bulk density and the crystallinity. In the case where the sintering temperature is higher than 950 ℃, non-uniform growth of particles may occur, and the volume capacity of the battery may be reduced because the size of the particles is excessively increased to reduce the amount of particles per unit area. The sintering temperature may be more preferably in the range of 800 to 900 deg.c in consideration of particle size control, capacity and stability of the prepared cathode active material particles and reduction of lithium-containing by-products.

According to the invention, the carbon in the carbon-incorporated precursor particles is combined with oxygen and converted into CO₂，CO₂Volatilize at a temperature of 450 ℃ to 700 ℃ and form a cavity at the carbon site. Therefore, the positive electrode active material prepared after the sintering process is performed contains a cavity formed by volatilization of carbon.

On the other hand, the positive electrode active material prepared according to the present invention has a cavity ratio of 5% to 20%, preferably 5% to 15%, which is an area ratio occupied by the cavity with respect to the cross-sectional area of the positive electrode active material particle. If the cavity ratio is less than 5%, the effects of increasing the surface area and improving the electrolyte permeation are insignificant, and if the cavity ratio is greater than 20%, the energy density is reduced, whereby the capacity and output characteristics may be reduced.

Positive electrode active material

Next, the positive electrode active material according to the present invention will be described.

The positive electrode active material of the present invention contains a lithium transition metal oxide, and when the distance from the center to the surface of the positive electrode active material particle is R, a cavity is contained in a region where the distance from the center of the particle is 0.3R to R, and the cavity ratio of the positive electrode active material is 5% to 20%.

When the distance from the center to the surface of the particles of the positive electrode active material prepared according to the above-described method of the present invention is R, the positive electrode active material contains a cavity in a region where the distance from the center of the particles is 0.3R or more.

Specifically, according to the present invention, since a carbon source is not added when forming the seed crystal of the precursor particle, but is added when the size of the precursor particle reaches 30% or more of the size of the finally prepared particle, the carbon is mainly introduced into a region having a distance of 0.3R or more from the center of the precursor particle, thereby forming a cavity, which is formed after the volatilization of the carbon, mainly in the region having a distance of 0.3R or more from the center of the precursor particle. If the cavity is included in the region where the distance from the center of the particle is 0.3R or more as in the present invention, the impregnation of the electrolytic solution can be smoothly performed, whereby an excellent effect of improving the battery performance can be obtained.

The positive electrode active material of the present invention has a cavity ratio of 5% to 20%, preferably 5% to 15%. The specific surface area of the cathode active material according to the present invention having the above-described cavity ratio is greater than that of the conventional cathode active material. Specifically, the cathode active material of the present invention may exhibit a Brunauer-Emmett-Teller (BET) specific surface area of 110% to 150% as compared to a conventional cathode active material not including a cavity. Thus, when the specific surface area is increased, the reaction area between the positive electrode active material and the electrolyte is maximized, whereby the output characteristics can be improved even if rapid charging is performed.

On the other hand, the diameter of the cavity may be 10nm to 1 μm, preferably 20nm to 500 nm. If the diameter of the cavity is too small, the effect of improving the specific surface area is insignificant, and if the diameter of the cavity is too large, physical properties such as energy density and tap density may be deteriorated.

Average particle diameter (D) of positive electrode active material according to the present invention₅₀) It may be 2 μm to 20 μm, preferably 4 μm to 20 μm.

According to the present invention, the tap density of the positive electrode active material may be 1.0g/cc to 4.0g/cc, preferably 1.5g/cc to 3.5 g/cc. If the positive electrode active material exhibits the above tap density, the reaction between the electrolyte and the positive electrode active material particles can be promoted due to the formation of the cavity while the energy density of the particles is not decreased.

Positive electrode

In addition, the present invention provides a positive electrode for a lithium secondary battery, comprising the positive electrode active material prepared by the above method.

Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer that is disposed on at least one surface of the positive electrode current collector and includes the positive electrode active material described above.

The positive electrode collector is not particularly limited as long as it has conductivity without causing adverse chemical changes in the battery, and for example: stainless steel, aluminum, nickel, titanium, and fired carbon; or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, and the like. In addition, the cathode current collector may generally have a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to improve adhesion of the cathode active material. For example, the positive electrode current collector may be used in various shapes such as a film, a sheet, a foil, a net, a porous body, a foam, a nonwoven fabric body, and the like.

The positive active material layer may include a conductive material and a binder, and a positive active material.

In this case, the content of the cathode active material may be 80 to 99 wt%, for example, 85 to 98 wt%, based on the total weight of the cathode active material layer. When the content of the positive electrode active material is within the above range, excellent capacity characteristics may be obtained.

In this case, the conductive material is used to provide conductivity to the electrode, and any conductive material may be used without particular limitation so long as it has electron conductivity without causing adverse chemical changes in the constructed battery. Specific examples of the conductive material may be the following: graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black and carbon fibers; powders or fibers of metals such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxide; or a conductive polymer such as a polyphenylene derivative, and any one thereof or a mixture of two or more thereof may be used. The content of the conductive material may be 1 to 30 wt% based on the total weight of the positive electrode active material layer.

The binder serves to improve the bonding between the positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples of the binder may be polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, Ethylene Propylene Diene Monomer (EPDM), sulfonated EPDM, Styrene Butadiene Rubber (SBR), fluororubber, or various copolymers thereof, and any one or a mixture of two or more thereof may be used. The binder may be contained in an amount of 1 to 30 wt% based on the total weight of the positive electrode active material layer.

The positive electrode may be manufactured according to a typical method of manufacturing a positive electrode, in addition to using the above-described positive electrode active material. Specifically, a composition for forming a positive electrode active material layer, which is prepared by dissolving or dispersing a positive electrode active material and optionally a binder and a conductive material in a solvent, is coated on a positive electrode current collector, and then a positive electrode may be manufactured by drying and rolling the coated positive electrode current collector. In this case, the types and amounts of the positive electrode active material, the binder, and the conductive material are the same as those described previously.

The solvent may be a solvent generally used in the art. The solvent may contain dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and any one of them or a mixture of two or more of them may be used. In view of the coating thickness and manufacturing yield of the slurry, the amount of the solvent may be sufficient if the solvent can dissolve or disperse the positive electrode active material, the conductive material, and the binder and can be made to have a viscosity that can provide excellent thickness uniformity during the subsequent coating for preparing the positive electrode.

Further, as another method, a cathode may be prepared by casting a composition for forming a cathode active material layer on a separate support and then laminating a film separated from the support on a cathode current collector.

Lithium secondary battery

In addition, the present invention can prepare an electrochemical device including the cathode. The electrochemical device may be, in particular, a battery or a capacitor, and more particularly, a lithium secondary battery.

The lithium secondary battery specifically includes a positive electrode, a negative electrode disposed in a manner facing the positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, wherein, since the positive electrode is the same as described above, a detailed description thereof will be omitted, and only the remaining configuration will be described in detail hereinafter.

In addition, the lithium secondary battery may further optionally include a battery container accommodating an electrode assembly of the positive electrode, the negative electrode and the separator, and a sealing member sealing the battery container.

In a lithium secondary battery, an anode includes an anode current collector and an anode active material layer disposed on the anode current collector.

The anode current collector is not particularly limited as long as it has high conductivity without causing adverse chemical changes in the battery, and for example: copper, stainless steel, aluminum, nickel, titanium and fired carbon; copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc.; and aluminum-cadmium alloys. In addition, the anode current collector may generally have a thickness of 3 to 500 μm, and as in the case of the cathode current collector, fine irregularities may be formed on the surface of the anode current collector to improve adhesion of the anode active material. For example, the anode current collector may be used in various forms such as a film, a sheet, a foil, a mesh, a porous body, a foam, and a nonwoven fabric body.

The anode active material layer selectively contains a binder and a conductive material in addition to the anode active material.

A compound capable of reversibly intercalating and deintercalating lithium may be used as the negative electrode active material. Specific examples of the anode active material may be the following: carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; (semi) metal-based materials capable of alloying with lithium, such as silicon (Si), aluminum (Al), tin (Sn), lead (Pb), zinc (Zn), bismuth (Bi), indium (In), magnesium (Mg), gallium (Ga), cadmium (Cd), Si alloys, Sn alloys, or Al alloys; metal oxides, e.g. SiO, which may or may not be doped with lithium_β(0<β<2)、SnO₂Vanadium oxide and lithium vanadium oxide; or a composite material comprising the (semi) metal-based material and a carbonaceous material, such as a Si-C composite material or a Sn-C composite material, and any one of them or a mixture of two or more thereof may be used. In addition, a metallic lithium thin film may be used as a negative electrode active material. Further, as the carbon material, both low crystalline carbon and high crystalline carbon may be used. Typical examples of the low crystalline carbon may be soft carbon and hard carbon, and typical examples of the high crystalline carbon may be irregular, planar, flaky, spherical or fibrous natural or artificial graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase carbon microbeads, mesophase pitch, and high-temperature sintered carbon such as petroleum or coal tar pitch-derived coke.

The content of the anode active material may be 80 parts by weight to 99 parts by weight based on 100 parts by weight of the total weight of the anode active material layer.

The binder is a component that facilitates the bonding between the conductive material, the active material, and the current collector, and is generally added in an amount of 0.1 to 10 parts by weight, based on 100 parts by weight of the total weight of the negative electrode active material layer. Examples of the binder may include acrylic copolymers, polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, Ethylene Propylene Diene Monomer (EPDM), sulfonated EPDM, styrene butadiene rubber, nitrile butadiene rubber, fluororubber, various copolymers thereof, and the like.

The conductive material is a component for further improving the conductivity of the anode active material, wherein the conductive material may be added in an amount of 10 parts by weight or less, such as 5 parts by weight or less, based on 100 parts by weight of the total weight of the anode active material layer. The conductive material is not particularly limited as long as it has conductivity without causing adverse chemical changes in the battery. For example, it is possible to use: graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black and thermal black; conductive fibers such as carbon fibers and metal fibers; carbon fluoride; metal powders such as aluminum powder and nickel powder; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxide; or a conductive material such as a polyphenylene derivative or the like.

For example, the anode active material layer may be prepared by coating a composition for forming an anode, which is prepared by dissolving or dispersing an anode active material and optionally a binder and a conductive material in a solvent, on an anode current collector and drying the coated anode current collector; or the anode active material layer may be prepared by casting the composition for forming the anode on a separate support and then laminating the film separated from the support on an anode current collector.

In the lithium secondary battery, a separator separates a negative electrode and a positive electrode and provides a moving path of lithium ions, wherein any separator may be used as the separator without particular limitation as long as it is generally used in the lithium secondary battery, and in particular, a separator having a high moisture-retaining ability to an electrolyte and a low resistance to migration of electrolyte ions may be used.

Specifically, it is possible to use: porous polymer films, for example, porous polymer films prepared from polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer, and ethylene/methacrylate copolymer; or have a laminated structure of two or more layers thereof. In addition, a typical porous nonwoven fabric, such as a nonwoven fabric formed of high-melting glass fibers or polyethylene terephthalate fibers, may be used. In addition, a coated separator including a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength, and may be selectively used in a single layer or a multi-layer structure.

In addition, the electrolyte used in the present invention may include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a melt-type inorganic electrolyte, which may be used during the preparation of a lithium secondary battery, but the electrolyte is not limited thereto.

Specifically, the electrolyte may include an organic solvent and a lithium salt.

Any organic solvent may be used without particular limitation so long as it can serve as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, the following substances may be used as the organic solvent: ester solvents such as methyl acetate, ethyl acetate, gamma-butyrolactone and epsilon-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), Ethyl Methyl Carbonate (EMC), Ethylene Carbonate (EC), and Propylene Carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitriles such as R-CN (where R is a linear, branched or cyclic C2-C20 hydrocarbyl group and may contain double bonds, aromatic rings or ether linkages); amides such as dimethylformamide; dioxolanes such as 1, 3-dioxolane; or sulfolane. Among these solvents, carbonate-based solvents are preferable, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ion conductivity and high dielectric constant and a low-viscosity chain carbonate-based compound (e.g., ethylene methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferable, which can improve the charge/discharge performance of the battery. In this case, when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte may be excellent.

Any compound may be used as the lithium salt without particular limitation so long as it can provide lithium ions used in the lithium secondary battery. In particular, LiPF may be used₆、LiClO₄、LiAsF₆、LiBF₄、LiSbF₆、LiAlO₄、LiAlCl₄、LiCF₃SO₃、LiC₄F₉SO₃、LiN(C₂F₅SO₃)₂、LiN(C₂F₅SO₂)₂、LiN(CF₃SO₂)₂、LiCl、LiI、LiB(C₂O₄)₂Etc. are used as the lithium salt. The lithium salt may be used in a concentration range of 0.1M to 2.0M. When the lithium salt concentration is within the above range, the electrolyte may have appropriate conductivity and viscosity to exhibit excellent performance, and lithium ions may be efficiently moved.

In the electrolyte, in order to improve the life characteristics of the battery, suppress the decrease in the capacity of the battery, and increase the discharge capacity of the battery, one or more additives may be further included, for example, halogenated alkylene carbonate-based compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (formal) glymes, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted compounds

Oxazolidinone, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, or the like. In this case, the additive may be contained in an amount of 0.1 to 5 parts by weight, based on 100 parts by weight of the total weight of the electrolyte.

The lithium secondary battery comprising the positive electrode active material according to the present invention as described above stably exhibits excellent discharge capacity, output characteristics, and life characteristics, so the lithium secondary battery is suitably used for: portable devices such as mobile phones, notebook computers, and digital cameras; and in the field of electric vehicles such as Hybrid Electric Vehicles (HEVs).

Therefore, according to another embodiment of the present invention, there are provided a battery module including the lithium secondary battery as a unit cell and a battery pack including the battery module.

The battery module or the battery pack may be used as a power source for at least one of medium-and large-sized devices: an electric tool; electric vehicles, including Electric Vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or an electrical power storage system.

The shape of the lithium secondary battery of the present invention is not particularly limited, but a cylindrical type, a prismatic type, a pouch type, or a coin type using a can may be used.

The lithium secondary battery according to the present invention may be used not only in a battery cell used as a power source for small-sized devices, but also as a unit cell in medium-and large-sized battery modules including a plurality of battery cells.

Modes for carrying out the invention

Hereinafter, the present invention will be described in detail based on specific examples. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

Example 1

Mixing NiSO₄、CoSO₄And MnSO₄A 2.2M solution containing a transition metal was prepared by mixing in distilled water with a molar ratio of nickel to cobalt to manganese of 85:5: 10.

The vessel containing the transition metal-containing solution was connected to a 20L reactor. Separately, a 4M NaOH solution and 15% NH were prepared₄The aqueous OH solution was connected to the reactor.

After 10L of deionized water is put in the water tankAfter 55 ℃ in a 20L reactor, the reactor was purged with nitrogen at a rate of 20L/min to remove dissolved oxygen from the water and create a non-oxidizing atmosphere in the reactor. Then, 100mL of a 20 wt% NaOH aqueous solution and 100 to 160mL of 15 wt% NH were added₄An aqueous OH solution was added to the reactor and the pH in the reactor was adjusted to 12.5.

Thereafter, a transition metal-containing solution was added to the reactor at a rate of 1L/hr, and NH was added₄Adding an OH aqueous solution into a reactor at a rate of 100 mL/h, adding an NaOH aqueous solution, and allowing the reaction solution to undergo a coprecipitation reaction for 120 minutes while maintaining the pH of the reaction solution at 11.5-12, to form seed crystals of the nickel-cobalt-manganese hydroxide particles.

Then, the addition rate of the aqueous NaOH solution to be added to the reactor was adjusted to maintain the pH at 11.5 and the reaction was carried out for 120 minutes, thereby growing nickel-cobalt-manganese particles to have an average particle diameter (D) of 3.5 μm₅₀)。

When the average particle diameter (D) of the nickel-cobalt-manganese particles₅₀) When the thickness became 3.5 μm, the transition metal-containing solution, the aqueous NaOH solution and NH were held₄The average particle diameter (D) of the carbon source as the amount of the OH aqueous solution added was adjusted at a rate of 50 mL/hr₅₀) The reaction solution was subjected to a coprecipitation reaction for 1500 minutes while a suspension of acetylene black (SB 50L derived from Denka co., ltd.) having a particle size of 50nm was added to the reactor to prepare a catalyst having an average particle size (D) of 10 μm₅₀) The positive electrode active material precursor of (1).

Then, a positive electrode active material precursor was mixed with LiOH in a molar ratio of Li to Me of 1.06:1, and the resulting mixture was sintered at 790 ℃ for 20 hours to prepare a positive electrode active material made of Li_1.06[Ni_0.85Co_0.05Mn_0.10]O₂The lithium transition metal oxide is shown.

Example 2

A lithium transition metal oxide was prepared in the same manner as in example 1, except that the carbon source was added at an addition rate of 70 mL/hr.

Example 3

A lithium transition metal oxide was prepared in the same manner as in example 1, except that 2000ppm of Zr was doped when the positive electrode active material precursor and LiOH were mixed and sintered.

Example 4

A lithium transition metal oxide was prepared in the same manner as in example 1, except that 2000ppm of Zr and 2000ppm of Al were doped when the positive electrode active material precursor and LiOH were mixed and sintered.

Comparative example 1

A lithium transition metal oxide was prepared in the same manner as in example 1, except that a separate carbon source was not added in the preparation of the cathode active material precursor.

Comparative example 2

A lithium transition metal oxide was prepared in the same manner as in example 1, except that nickel-cobalt-manganese hydroxide was grown up to the average particle diameter (D)₅₀) After the carbon source had reached 3.5 μm, the average particle size (D) as a carbon source was added together with the transition metal-containing solution at a rate of 46 mL/hr from the start of the reaction without adding the aqueous carbon source solution₅₀) A suspension of 50nm acetylene black (SB 50L from Denka co., ltd.) and the reaction solution was subjected to a coprecipitation reaction for 1620 minutes, thereby preparing a positive electrode active material precursor. In this case, the amount of carbon source added was adjusted to be the same as the amount of carbon source added in example 1.

Comparative example 3

A lithium transition metal oxide was prepared in the same manner as in example 1, except that the carbon source was added at a rate of 120 mL/hr.

Comparative example 4

Except that the average particle diameter (D) as a carbon source was added₅₀) A lithium transition metal oxide was prepared in the same manner as in example 1, except for being a suspension of 1.2 μm acetylene black (SB 50L derived from Denka co.

Experimental example 1: characteristic of positive electrode active material particlesConfirmation

(1) Particle size of positive electrode active material

Each of the lithium transition metal oxides prepared in examples 1 to 4 and comparative examples 1 to 4 above was pulverized for 10 minutes by using a simple mixer (ACM), then introduced into a laser diffraction particle size meter (e.g., Microtrac MT 3000) and irradiated with ultrasonic waves having a frequency of about 28kHz and an output of 60W, and then an average particle diameter (D) of 50% based on the cumulative distribution of the number of particles was calculated from the particle diameters in the meter (D)₅₀). The results are shown in table 1 below.

(2) Rate of cavitation

The cavity ratios of the particles prepared in examples 1 to 4 and comparative examples 1 to 4 above were measured by cross-sectional analysis of the particles using a Focused Ion Beam (FIB), respectively. Specifically, the lithium transition metal oxide particles in examples 1 to 4 and comparative examples 1 to 4 were cut using FIB, and a cross-sectional photograph was taken by a Scanning Electron Microscope (SEM), and then the cross-sectional photograph was analyzed to determine the ratio of the area of the entire cavity to the cross-sectional area of the lithium transition metal oxide particles. The measurement results are shown in table 1 below.

(3) BET specific surface area (m)²/g)

The specific surface area of the lithium transition metal oxide particles was measured by the brunauer-emmett-teller (BET) method, specifically, the specific surface area was calculated from the nitrogen adsorption amount at a liquid nitrogen temperature (77K) using the bessel orp-mini II of Bell Japan inc, the calculated value was compared with the value of comparative example 1, and the BET specific surface area compared with comparative example 1 was shown in table 1 in a percentage [ (sample/comparative example 1) × 100 ].

(4) Tap density (g/cc)

After 50g each of the positive electrode active material precursors obtained in examples 1 to 2 and comparative examples 1 to 4 was filled in a 100cc container, the tap density of lithium transition metal oxide particles was measured by using a tap density tester (KYT-4000 from Seishin co. The measurement results are shown in table 1 below.

[ Table 1]

As shown in table 1 above, it can be confirmed that the cathode active materials prepared according to the method of the present invention in examples 1 and 2 and having a cavity ratio in the range of 5% to 20% have a higher specific surface area than the cathode active material in comparative example 1 and have a tap density similar to that of comparative example 1.

In contrast, it was confirmed that in the case of comparative example 2 in which the carbon source was added from the beginning of the reaction, the growth of the particles was suppressed due to the carbon source, and thus final cathode active material particles having a small size were formed, and the cathode active materials having a cavity rate of more than 20% in comparative examples 3 and 4 had a decreased tap density.

Experimental example 2

Lithium secondary batteries were prepared by using the positive electrode active materials prepared in examples 1 to 4 and comparative examples 1 to 4 above, and the capacity and resistance characteristics at high rate of various lithium secondary batteries including the positive electrode active materials of examples 1 to 4 and comparative examples 1 to 4 were evaluated.

Specifically, each of the positive electrode active materials prepared in examples 1 to 4 and comparative examples 1 to 4, a carbon black conductive material (FX 35 obtained from Denka co., ltd.) and a polyvinylidene fluoride (PVdF) binder (KF 9700 obtained from Kureha co., ltd.) were mixed in a solvent of N-methylpyrrolidone (NMP) at a weight ratio of 96.5:1.5:2.0 to prepare a positive electrode slurry. The positive electrode slurry was coated on one surface of an aluminum current collector, dried at 130 ℃, and then rolled to prepare a positive electrode.

On the other hand, artificial graphite, a carbon black conductive material (SUPER C-65), and an acrylic binder (BM-L302 from Zeon Corporation) were mixed at a weight ratio of 96:1:3, and added to water as a solvent, thereby preparing anode active material slurry. The negative active material slurry was coated on a copper foil having a thickness of 20 μm, dried, and then rolled to prepare a negative electrode.

Various lithium secondary batteries were manufactured by preparing an electrode assembly by disposing a polyethylene separator between the positive and negative electrodes prepared as described above, placing the electrode assembly in a battery case, and then injecting an electrolyte into the case. The charge/discharge efficiency of the lithium secondary batteries prepared in the above examples 1 to 4 and comparative examples 1 to 4 was evaluated, and the results are shown in table 2 below.

Specifically, the lithium secondary batteries of examples 1 to 4 and comparative examples 1 to 4 were charged to 4.2V at 25 ℃ at a constant current of 0.1C and discharged to 3V at a constant current of 0.1C, and then charge and discharge characteristics in the first cycle were observed. Thereafter, the change of the capacity retention rate with the C rate was measured by changing the discharge condition to 2.0C, and the 10-second resistance was confirmed by discharging at a constant current of 2.6C in a state of charge (SOC) of 50%. The results are shown in table 2 below.

[ Table 2]

As shown in table 2 above, it was confirmed that the lithium secondary batteries of examples 1 to 4 of the present invention have more excellent capacity and resistance characteristics according to the change in C-rate than the lithium secondary batteries of comparative examples 1 to 4. In particular, it was confirmed that comparative example 2 shows a cavity rate similar to that of example 1 by adjusting the addition amount to add the same amount of carbon source as that of example 1, but since the cavity exists not only outside the positive electrode active material but also inside due to the addition of the carbon source from the start of the reaction, the impregnation of the electrolyte solution is deteriorated as compared to examples 1 to 4, thereby deteriorating the capacity, the resistance characteristics, and the like.

Experimental example 3

The energy densities per unit volume of the lithium secondary batteries of examples 1 to 4 and comparative examples 1 to 4 prepared in experimental example 2 above were evaluated, and the ratio to the energy density of example 1 was confirmed. The energy density per unit volume was determined by the calculation formula of the following equation (1).

[ equation 1]

Energy density (Wh/L) ═ nominal voltage × 0.1C discharge capacity/cell volume of secondary battery

[ Table 3]

	Ratio of energy Density (Wh/L) to example 1 (%)
		Example 1	100 (ref)
Example 2	89.4
		Example 3	99.8
Example 4	99.7
		Comparative example 1	99.7
Comparative example 2	98.0
		Comparative example 3	68.0
Comparative example 4	74.7

From table 3 above, it can be confirmed that comparative examples 3 and 4 having a cavity ratio of more than 20% have a significantly reduced energy density. In contrast, it can be confirmed that examples 1,3 and 4 and comparative example 2 having a cavity ratio of 5% to 15% have an energy density equal to or better than comparative example 1 containing less cavities. It is shown that example 2 having a cavity rate of more than 15% has a slightly decreased energy density.

Experimental example 4

The positive electrode active material precursor particles and the positive electrode active material prepared in example 1 and the positive electrode active material precursor particles and the positive electrode active material prepared in comparative example 1 were cut by using FIB, and each cross-sectional image was photographed by SEM.

Fig. 1 shows a cross-sectional SEM photograph of the cathode active material precursor particles of example 1, and fig. 2 shows a cross-sectional SEM photograph of the cathode active material of example 1.

In addition, fig. 3 shows a cross-sectional SEM photograph of the cathode active material precursor particles prepared in comparative example 1, and fig. 4 shows a cross-sectional SEM photograph of the cathode active material prepared in comparative example 1.

It can be confirmed from fig. 2 that cavities of several tens nanometers to several hundreds nanometers are formed on the positive electrode active material particles of example 1, and it can be seen that the total area forming the cavities accounts for about 10% of the entire cross-sectional area of the positive electrode active material particles.

In contrast, in the case of the positive electrode active material particles of comparative example 1, cavities were hardly formed, as shown in fig. 4.

Claims

1. A method of preparing a positive electrode active material, the method comprising:

a first step of adding a reaction solution to a reactor to form seeds of precursor particles, the reaction solution including a transition metal-containing solution containing at least one of nickel, cobalt, and manganese, an ammonium ion-containing solution, and an aqueous alkaline solution;

by growing on the precursor particles up to the average particle diameter (D) of the precursor particles₅₀) For the precursor particles finally preparedAverage particle diameter (D)₅₀) A second step of adding a carbon source to the reactor at 30% of the size of (a) to prepare carbon-introduced precursor particles; and

a third step of mixing the carbon-introduced precursor particles with a lithium raw material and sintering the mixture at a temperature of 750 to 950 ℃ to prepare positive electrode active material particles,

wherein the carbon introduced into the precursor particles is volatilized by the sintering of the third step to form cavities in the positive electrode active material particles, and a cavity ratio of the positive electrode active material is 5% to 20%.

2. The method of claim 1, wherein the finally prepared precursor particles have an average particle diameter (D) of 2 μm to 20 μm₅₀)。

3. The method of claim 1, wherein the average particle diameter (D) of the precursor particles when in the second step₅₀) When the carbon source becomes 1 to 7 μm, the carbon source is added to the reactor.

4. The method of claim 1, wherein the carbon source comprises at least one carbon-based material selected from the group consisting of: carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fibers, and carbon nanotubes.

5. The method according to claim 4, wherein the carbon source is added in such a manner that the carbon-based material is 20 vol% or less with respect to 100 vol% of the entire precursor particles.

6. The method according to claim 4, wherein the carbon-based material has an average particle diameter (D) of 10nm to 1 μm₅₀)。

7. The method according to claim 1, wherein the positive electrode active material has a cavity rate of 5% to 15%.

8. A positive electrode active material comprising a lithium transition metal oxide, wherein when a distance from a center to a surface of a particle of the positive electrode active material is R, the positive electrode active material comprises a cavity in a region where the distance from the center of the particle is 0.3R or more, and

the positive electrode active material particles have a cavity rate of 5% to 20%.

9. The positive electrode active material according to claim 8, wherein the cavity has a diameter of 10nm to 1 μm.

10. The cathode active material according to claim 8, wherein the cathode active material has an average particle diameter (D) of 2 to 20 μm₅₀)。

11. A positive electrode for a lithium secondary battery, comprising the positive electrode active material according to claim 8.

12. A lithium secondary battery comprising the positive electrode for a lithium secondary battery according to claim 11.